Amazon's latest push into fast grocery delivery had a surprising beneficiary: Instacart. Both Amazon and Instacart are competing to deliver your weekly supermarket haul — or maybe a fill-in trip for milk. Last month, Amazon said it had expanded same-day delivery on perishable groceries, such as meat and frozen foods, to 1,000 cities and towns, with plans to reach more than double that many by year's end.That's a threat to Instacart, which offers delivery and other services to grocers like Aldi and Costco. ...